Conventionally, imaging devices such as monitoring cameras have been used to monitor an object entering in a monitoring target region. In addition, there has been examined not a technique for manned monitoring by monitoring personnel but a technique for automatic monitoring by a video monitoring device or the like. A video monitoring device based on such a technique can implement the function of recording only video images in which a moving object appears by using the detection results, the function of displaying an alarm icon on a display device, and the function of sounding a beeper or the like to call attention of the monitoring personnel, which are useful in reducing the burden of monitoring operations in which constant checking has been conventionally required.
As an example of a technique for automatically detecting an object entering in a monitoring target region, there is widely known a monitoring technique using a background difference method (for example, refer to Patent Literature 1). According to an object detection method by using background difference, the difference in luminance (or pixel value) between an input image obtained from an imaging device or the like and a background image as a reference in which no object to be detected is seen is calculated. When there is any change region where the calculated detection value is larger than a predetermined threshold, it is determined that an object to be detected exists or possibly exists in the change region, and the change region is monitored.
In addition, for detecting a state in which a dangerous object or the like is left to stand, there is not only a method by merely detecting changes in the video image but also a method by detecting a motionless region in which no changes from the background image are constantly detected, as a left object.